In some vapour compression systems an ejector is arranged in a refrigerant path, at a position downstream relative to a heat rejecting heat exchanger. Thereby refrigerant leaving the heat rejecting heat exchanger is supplied to a primary inlet of the ejector. Refrigerant leaving an evaporator of the vapour compression system is supplied to a secondary inlet of the ejector.
An ejector is a type of pump which uses the Venturi effect to increase the pressure energy of fluid at a suction inlet (or secondary inlet) of the ejector by means of a motive fluid supplied to a motive inlet (or primary inlet) of the ejector. Thereby, arranging an ejector in the refrigerant path as described will cause the refrigerant to perform work, and thereby the power consumption of the vapour compression system is reduced as compared to the situation where no ejector is provided.
An outlet of the ejector is normally connected to a receiver, in which liquid refrigerant is separated from gaseous refrigerant. The liquid part of the refrigerant is supplied to the evaporator, via an expansion device. The gaseous part of the refrigerant may be supplied to a compressor. Thereby the gaseous part of the refrigerant is not subjected to the pressure drop introduced by the expansion device, and the work required in order to compress the refrigerant can therefore be reduced.
If the pressure inside the receiver is high, the work required by the compressors in order to compress the gaseous refrigerant received from the receiver is correspondingly low. On the other hand, a high pressure inside the receiver has an impact on the liquid/gas ratio of the refrigerant in the receiver to the effect that less gaseous and more liquid refrigerant is present, and a too high pressure inside the receiver is therefore not desirable, as it forces the pressure inside the heat rejecting heat exchanger to be even higher, thereby decreasing the efficiency of the vapour compression system. Furthermore, at low ambient temperatures, the efficiency of the vapour compression system is normally improved when the pressure inside the heat rejecting heat exchanger is relatively low.
Accordingly, a suitable pressure level inside the receiver must be defined, which balances the work required by the compressor and other system requirements, as described above. Furthermore, it is desirable to supply as much refrigerant as possible from the outlet of the evaporator to the secondary inlet of the ejector, because the pressure of the refrigerant being supplied to the compressors is higher, thereby reducing the amount of work required by the compressors in order to compress the refrigerant. However, the amount of refrigerant being supplied from the outlet of the evaporator to the secondary inlet of the ejector must not be so large that the pressure of the refrigerant leaving the evaporator decreases below an acceptable level. The amount of refrigerant being sucked into the secondary inlet of the ejector depends, inter alia, upon the pressure difference between the pressure of the refrigerant leaving the heat rejecting heat exchanger and the pressure of the refrigerant leaving the ejector, i.e. the pressure inside the receiver.
US 2012/0167601 A1 discloses an ejector cycle. A heat rejecting heat exchanger is coupled to a compressor to receive compressed refrigerant. An ejector has a primary inlet coupled to the heat rejecting heat exchanger, a secondary inlet and an outlet. A separator has an inlet coupled to the outlet of the ejector, a gas outlet and a liquid outlet. The system can be switched between first and second modes. In the first mode refrigerant leaving the heat absorbing heat exchanger is supplied to the secondary inlet of the ejector. In the second mode refrigerant leaving the heat absorbing heat exchanger is supplied to the compressor.